Optical waveguide material and optical waveguide

ABSTRACT

The invention provides an optical waveguide material whose refractive index can be tailored without changing the ratio of Ta and Nb. An optical waveguide of this invention comprising an under-clad layer  1  and a core  2  that is formed on the under-clad layer  1  and has a higher refractive index than that of the under-clad layer  1  is shown. For example, KTN (KTa 1−x Nb x O 3 ) is used as the core  2 , and a material that is obtained by substituting at least one element selected from the group consisting of Zr, Hf, and Sn for a portion of one element of the constituent elements of KTN and has the same perovskite type crystal structure as KTN is used as the clad. The refractive index of KTN can be reduced considerably, and this controllability widens the degree of freedom in the design of optical waveguide devices.

TECHNICAL FIELD

The present invention relates to an optical waveguide material and anoptical waveguide, and more specifically, to an optical waveguidematerial used for optical waveguide devices for optical communications,its manufacture method, and an optical waveguide using this opticalwaveguide material.

BACKGROUND ART

A structure in which a high refractive index material is surrounded by amaterial whose refractive index is lower than that of the highrefractive index material acts as an optical waveguide that confinesoptical energy of light in the high refractive index region and itsvicinity, and transmits the light in itself. A part whose refractiveindex is high is called a core, and a part whose refractive index is lowis called a clad or cladding. The optical waveguide is applied tovarious optical components including those for optical communications.

Potassium tantalate niobate (KTa_(1−x)Nb_(x)O₃) is known as one exampleof the optical waveguide materials. KTa_(1−x)Nb_(x)O₃ (hereinafterreferred to as KTN) is a material having the perovskite type crystalstructure. FIG. 1 shows the unit cell of KTN crystal. When a simplecubic lattice having potassium ions on its lattice points is considered,an ion of tantalum or niobium is placed at its body center position, andoxygen ions are placed at its face center positions. KTN is a crystalmaterial with a very large electro-optic effect that the refractiveindex varies upon application of an electric field (for example, seePatent Document 1). Moreover, since its refractive index will also varywhen its composition is changed, an optical waveguide can be constructedwith KTN by manufacturing a core and a clad each having a differentcomposition.

The optical waveguide using KTN can modulate the phase of light beingtransmitted therein by means of the electro-optic effect by providingappropriate electrodes. Therefore, KTN can be used to manufactureoptical components, such as an optical modulator etc., as is the case ofLiNbO₃ whose development for optical components has been preceded. SinceKTN has a remarkably large electro-optic effect as compared with LiNbO₃,KTN has the advantage of enabling optical components of higherperformance in terms of low voltage operation etc. to be obtained.

In addition, K_(1−y)Li_(y)Ta_(1−x)Nb_(x)O₃ (hereinafter referred to asKLTN) obtained by substituting Li for a portion of K of KTN is also apromising material that has the same perovskite type crystal structureas KTN and has a larger electro-optic effect than that of KTN.

However, with a change in a ratio of Ta and Nb in KTN and KLTN describedabove, the refractive index will vary, and at the same time theelectro-optic coefficient and permittivity will change. For this reason,it was difficult to optimize characteristics of optical components bychanging these parameters independently.

For example, when a refractive index difference between the core and theclad is set to 0.011 or more in order to enhance the performance of anoptical waveguide element, the permittivities of the two members make alarge difference. As a result, an electric field cannot be effectivelyapplied to the optical waveguide, and hence an optical component makingfull use of a large electro-optic effect cannot be manufactured.

KTaO₃ (hereinafter referred to as KT) has a small electro-optic effectcompared with KTN and KLTN described above, but is known as an opticalwaveguide material of a high refractive index and is transparent forlight of wavelengths down to 350 nm. KT is a crystal of an end member ofKTN and KLTN, having the same perovskite type crystal structure as KTNand KLTN. That is, if a portion of Ta of KT is substituted for by Nb,the partially substituted KT becomes KTN. If a portion of Ta of KT issubstituted for by Nb and a portion of K is substituted for by Li, thepartially substituted KT becomes KLTN. Therefore, KT is used as asubstrate material on which KTN or KLTN crystal layers are grown. If anoptical waveguide is constructed using KT, its refractive index cannotbe controlled by changing a ratio of Ta and Nb, because KT does notinclude Nb.

The object of this invention is to provide an optical waveguide materialwhose refractive index can be tailored without changing the ratio of Taand Nb.

Patent Document 1: Japanese Patent application laid-open No. 2003-35831

DISCLOSURE OF THE INVENTION

In order to achieve this object, this invention provides an opticalwaveguide material that is made up of a crystal that has a compositionof KTaO₃ (KT) wherein at least one element selected from the groupconsisting of Zr, Hf, and Sn substituted for a portion of one element ofthe constituent elements of KT and that has the same perovskite typecrystal structure as KT.

The optical waveguide material may have a composition obtained byreplacing a portion of one element of the constituent elements of KTwith other element. Moreover, the optical waveguide material may be acrystal (KTa_(1−x)Nb_(x)O₃:KTN:0<=x<=1) having a composition obtained bysubstituting Nb for a portion of Ta of KT. Furthermore, the opticalwaveguide material may be a crystal(K_(1−y)Li_(y)Ta_(1−x)Nb_(x)O₃:KLTN:0<=x<=1) having a compositionobtained by substituting Li for a portion of K of KTN.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a unit cell of KTN crystal.

FIG. 2 is a perspective view showing a structure of the opticalwaveguide to which the optical waveguide material of this invention isapplied.

FIG. 3 is a diagram showing the relationship between the amount ofaddition of Zr and the refractive index.

FIG. 4 is a diagram showing the refractive index of KTN crystal obtainedwith the amount of addition of 0.5 mol %.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of this invention will be described below referring to thedrawings.

FIG. 2 is a perspective view showing a structure of the opticalwaveguide to which the optical waveguide material of this invention isapplied. The basic structure of the optical waveguide is a ridge typeoptical waveguide, and has electrodes for applying an electric field asone example of an external field applying mechanism. A substrate acts asan under-clad layer 1, on which a core 2 having a higher refractiveindex than that of the substrate is formed. An electrode 4 for applyingan electric field is formed on the upper side of the core 2.Incidentally, an over-clad layer (cover layer) 6 that covers the core 2and has a lower refractive index than that of the core 2 may be formedto make an embedded optical waveguide. All of the under-clad layer 1,the core 2, and the over-clad layer 6 are made of any one of materialsof KT, KTN, and KLTN.

Here, even when the core and the clad are made up of a crystal of thesame composition, the refractive index difference between the core andthe clad can be controlled by the amount of at least one elementselected from the group consisting of Zr, Hf, and Sn that hassubstituted for a portion of one element of the constituent elements ofits original crystal. This is because the refractive index of any one ofKT, KTN, and KLTN can be lowered by substituting at least one elementselected from the group consisting of Zr, Hf, and Sn for a portion ofone element of the constituent elements of that crystal. The amount ofdecrease in the refractive index is proportional to the amount of theelements Zr, Hf, and Sn that was used for substitution. Therefore, inthe optical waveguide of this example, the refractive index differencebetween the core and the clad can be controlled without changing theratio of Ta and Nb.

By this structure, the optical input signal 3 is launched from anoptical-waveguide end face, and transmitted in the core 2. The core 2produces a refractive index change by an external voltage signal appliedto the core 2 with the electrode 4. By this arrangement, a propagatingoptical signal is phase-modulated when passing through the core 2. Theoptical signal that was modulated is taken out from a counter end faceto the outside as an optical output signal 5. It is obvious that thestructure of the optical waveguide described above is just to show oneexample, and the optical waveguide material of the invention is notapplied only to this structure of the optical waveguide.

In the one example of this invention, at least one of the under-cladlayer 1, the core 2, and the over-clad layer 6 uses a material that is acrystal of one composition selected from among compositions of KT, KTN,and KLTN wherein at least one element selected from the group consistingof Zr, Hf, and Sn substituted for a portion of one element of theconstituent elements of the selected composition and has the sameperovskite crystal structure as KT. Hereafter, the optical waveguidematerial of this invention will be described more concretely.

It is well known that a refractive index of an arbitrary, transparentmedium can be controlled by adding a very small amount of an additive tothe medium. While it is relatively easy to add an additive to polymermaterials, glasses, ceramics, etc. it is usually not easy to add anadditive to single crystals. This is because when a signal crystal isgrown, impurities that inhibit the formation of a stable crystalstructure are eliminated.

The single crystals of KT, KTN, and KLTN can be grown by addingpotassium oxide (K₂O) as a flux to raw materials, melting them, andcooling down the melt slowly. In order to add other element(s) to one ofthese crystals, as a general case, the intended element is meltedtogether with raw materials of the crystal, and the crystal is grownfrom the melt of them.

A ratio k=x/X of a mole concentration x of an additive in the crystal toa mole concentration X of the additive in the melt is termed thesegregation coefficient. As mentioned above, most impurities are hard tobe incorporated in crystals, and accordingly k's are very small. Forexample, when adding copper (Cu), k is roughly in the range from 10⁻³ to10⁻² depending on conditions. This fact becomes a main reason why it isdifficult to change the refractive index by 0.0001 or more (equivalentto a relative refractive index change of 0.05% or more). Generally, itis not easy to predict which additive has a better tendency to beincorporated into a certain transparent medium, and whether thatadditive has an effect to modify the refractive index.

The inventors of this invention have found that zirconium (Zr), hafnium(Hf), and tin (Sn) are easy to be incorporated into single crystals ofKT, KTN, and KLTN system materials as compared with other additives, andthat the segregation coefficients k's for these cases exceed 1.0. Forthis reason, when one or more of these elements are added, the additioncan cause a large refractive index decrease as much as 0.01.

Refractive indices of KT (including KTN and KLTN) system waveguidematerials are approximately 2.2, and so this refractive index change is0.5% when expressed at a ratio to the refractive index. This iscomparable to a refractive index difference between a core and a clad ofthe common optical waveguide. That is, putting this invention intopractice enables the refractive index of an optical waveguide to becontrolled sufficiently, and accordingly it becomes possible tomanufacture a device that is optimized in terms of both electricproperties, such as permittivity, and the refractive index difference.As a result, the degree of freedom in the design of optical waveguidedevices using these single crystals can be widened, and hence theirperformance in terms of the operating voltage, the band, etc. can beimproved.

EXAMPLE 1

Powders of potassium carbonate (K₂CO₃) and tantalum oxide (Ta₂O₅) aremixed in a mole ratio of 3:2, charged in a platinum crucible, heated to1400° C. in an electric furnace, and made to react with each othersufficiently. Then, the melt is cooled slowly to 800° C. in ten days togrow a crystal of KTaO₃. The refractive index of the taken-out singlecrystal is 2.1542 at 1550 nm wavelength.

Similarly, ZrO₂ of a mole ratio of 1% is mixed with the mixed powders oftantalum oxide and potassium carbonate, and a crystal of KTaO₃ is grown.The refractive index of the KTaO₃ single crystal to which Zr was addedis 2.1407 at 1550 nm wavelength. Thereby, a refractive index differenceof 0.0135 can be created.

FIG. 3 shows the relationship between the amount of addition of Zr andthe refractive index. This graph has a horizontal axis representing theamount of ZrO₂ mixed into raw materials of K₂CO₃ and Ta₂O₅ and avertical axis representing the refractive index of the obtained singlecrystal. The graph shows that with increasing amount of addition ofZrO₂, the refractive index decreases. Incidentally, the segregationcoefficient of Zr is 2.6.

Next, the single crystal grown from the raw materials to which 1 mol %ZrO₂ was added is sliced to produce a substrate. On this substrate, anoptical-waveguide core part made up of a KTaO₃ single crystal withnothing added is formed, and then a KTaO₃ single crystal film to which 1mol % ZrO₂ was added is grown as a clad in such a way as to cover thecore part. The manufactured optical waveguide can transmit light, thelight being confined in the core part and its vicinity as designed.

EXAMPLE 2

A part of tantalum oxide in the raw materials in the example 1 isreplaced with niobium oxide (Nb₂O₅), and a crystal of KTN is grown. As aresult, with addition of 1 mole % ZrO₂, the refractive index is deceasedby about 0.01, regardless of a value of composition ratio x ofKTa_(1−x)Nb_(x)O₃.

EXAMPLE 3

Further, a crystal of KLTN is grown by replacing a part of potassiumcarbonate in the raw materials in the example 2 with lithium carbonate.By adding 1 mol % ZrO₂, the refractive index can be lowered by about0.01. Alternatively, also in the case where crystal growth is performedby replacement of potassium carbonate with sodium carbonate instead oflithium carbonate, the refractive index difference can be created as adecrease of 0.01 by adding ZrO₂ similarly.

EXAMPLE 4

HfO₂ is added instead of ZrO₂ for each of KT, KTN, and KLTN. Theaddition of 1 mol % HfO₂ can decrease the refractive index by 0.015.Similarly, when adding SnO₂, the refractive index can be decreased by0.006 for the same amount of addition.

FIG. 4 shows the refractive index of KTN crystal obtained with an amountof addition of 0.5 mol %. As is clear from FIG. 4, the additives ofZrO₂, SnO₂, and HfO₂ effectively decrease the refractive index,respectively, whereas other additives (NiO, MoO₃, Al₂O₃, and ZnO) thanthat of the three additives can hardly change the refractive index ofKTN.

INDUSTRIAL APPLICABILITY

This invention relates to the optical waveguide material used foroptical components for optical communications and the like and canprovide the optical waveguide material whose refractive index can betailored without changing the ratio of Ta and Nb. Moreover, thisinvention can widen the degree of freedom in the design of opticalwaveguide devices using these single crystals and improve theirperformance in terms of the operating voltage, the band, etc.

1. An optical waveguide material comprising a crystal having acomposition of K_(1−y)Li_(y)Ta_(1−x)Nb_(x)O₃ (KLTN:0<x, y<1); wherein atleast one element selected from the group consisting of Zr, Hf, and Snsubstitutes for a portion of one element of the constituent elements ofKLTN, the segregation coefficients of the at least one element exceeds1.0, the crystal has the same perovskite type crystal structure as KLTN,and a refractive index decrease is controlled by the amount of the atleast one element.
 2. An optical waveguide comprising a core and a cladwhose refractive index is lower than that of the core; wherein a corematerial and a clad material are crystals that have a composition ofK_(1−y)Li_(y)Ta_(1−x)Nb_(x)O₃ (KLTN:0<x, y<1), at least one elementselected from the group consisting of Zr, Hf, and Sn substitutes for aportion of one element of the constituent elements of KLTN, thesegregation coefficients of the at least one element exceeds 1.0, thecrystal has the same perovskite type crystal structure as KLTN, and therefractive index difference between the core and the clad is controlledby the amount of the at least one element selected from the groupconsisting of Zr, Hf, and Sn.